Fabricating transistors with implanting dopants at first and second dosages in the collector region to form the base region

ABSTRACT

An integrated circuit includes a transistor that has an collector region, a base region laterally surrounded by the collector region, and an emitter region laterally surrounded by the base region. A silicide layer on the emitter region is laterally spaced apart from the base region by an unsilicided ring. The emitter region is laterally spaced apart from a base contact region that may be covered by a dielectric layer such as a gate oxide layer.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/859,292, filed Dec. 29, 2017, and references U.S. patent applicationSer. No. 15/824,665, which was filed Nov. 28, 2017, and titled“FABRICATING TRANSISTORS WITH A DIELECTRIC FORMED ON A SIDEWALL OF AGATE MATERIAL AND A GATE OXIDE BEFORE FORMING SILICIDE,” both of whichare hereby incorporated herein by reference in their entireties.

BACKGROUND

Bipolar junction transistors are often used in many high performanceanalog applications to amplify or buffer analog signals. In suchapplications, it is desirable for bipolar junction transistors toexhibit a relatively high transistor beta, β, and in particular, arelatively high beta Early voltage product, βV_(A). It is also desirablefor bipolar junction transistors to exhibit relatively low 1/f noise andpopcorn noise, and yet have a relatively high beta or a relatively highbeta Early voltage product.

SUMMARY

In accordance with a first set of embodiments, a method to fabricate atransistor, the method comprising: implanting dopants in a semiconductorto form a collector region having majority carriers of a first type;implanting dopants with a first dosage and implanting dopants with asecond dosage in the collector region to form a base region havingmajority carriers of a second type, wherein the second dosage is at alower energy than the first dosage; forming a gate oxide on the baseregion; forming a gate material on the gate oxide; forming the gatematerial and the gate oxide to leave uncovered an emitter area of thebase region; and implanting dopants in the emitter area to form anemitter region having majority carriers of the first type.

In accordance with the first set of embodiments, the method furthercomprises: forming a dielectric to cover a first area of the emitterregion and a first sidewall of the gate material and the gate oxide, andto leave uncovered a second area of the emitter region; depositing ametal over the dielectric and the second area of the emitter region; andannealing the semiconductor to form silicide in the second area of theemitter region, wherein forming the dielectric to cover the first areaof the emitter region and the first sidewall of the gate material andthe gate oxide is performed before annealing the semiconductor to formsilicide.

In accordance with the first set of embodiments, the method furthercomprises: forming the gate material and the gate oxide to leaveuncovered a base contact drain area of the base region; and implantingdopants in the base contact area of the base region to form a basecontact region having majority carriers of the second type.

In accordance with the first set of embodiments, the method furthercomprises: forming the dielectric to cover a first area of the basecontact region and a second sidewall of the gate material and the gateoxide, and to leave uncovered a second area of the base contact region;depositing the metal over the second area of the base contact region;and when annealing the semiconductor, forming silicide in the secondarea of the base contact region, wherein forming the dielectric to coverthe first area of the base contact region and the second sidewall of thegate material and the gate oxide is performed before annealing thesemiconductor to form silicide.

In accordance with the first set of embodiments, in the method, thedielectric comprises silicon dioxide.

In accordance with the first set of embodiments, in the method, the gatematerial comprises polysilicon.

In accordance with the first set of embodiments, in the method, themetal comprises tungsten.

In accordance with the first set of embodiments, the method furthercomprises removing the metal that has not formed the silicide.

In accordance with the first set of embodiments, in the method, themajority carriers of the first type are holes and the majority carriersof the second type are electrons.

In accordance with the first set of embodiments, the method furthercomprises: implanting dopants in the collector region to form a wellhaving majority carriers of the first type; implanting dopants in thewell to form a collector contact region having majority carriers of thefirst type; depositing the metal over the collector contact region; andwhen annealing the semiconductor, forming silicide in the collectorcontact region.

In accordance with a second set of embodiments, a method to fabricate atransistor, the method comprising: implanting dopants in a semiconductorto form a collector region having majority carriers of a first type;implanting dopants with a first dosage and implanting dopants with asecond dosage in the collector region to form a base region havingmajority carriers of a second type, wherein the second dosage is at alower energy than the first dosage; growing a gate oxide on thesemiconductor; depositing a gate material on the gate oxide; etching thegate material and the gate oxide to expose an emitter area of the baseregion surrounded by a first sidewall of the gate material and the gateoxide, and to expose a base contact area of the base region, the basecontact area surrounding a second sidewall of the gate material and thegate oxide; implanting dopants in the emitter area to form an emitterregion having majority carriers of the first type; implanting dopants inthe base contact area of the base region to form a base contact regionhaving majority carriers of the second type; depositing a dielectricover the semiconductor; etching the dielectric to cover the firstsidewall of the gate material and the gate oxide, the dielectric afteretching exposing a portion of the semiconductor; depositing metal overthe dielectric and the exposed portion of the semiconductor; andannealing the semiconductor to form silicide in the exposed portion ofthe semiconductor.

In accordance with the second set of embodiments, the method furthercomprises etching the dielectric to cover the second sidewall of thegate material and the gate oxide.

In accordance with the second set of embodiments, in the method, thedielectric comprises silicon dioxide, the method further comprisingremoving the metal from the dielectric.

In accordance with the second set of embodiments, in the method, themajority carriers of the first type are holes and the majority carriersof the second type are electrons.

In accordance with a third set of embodiments, a transistor comprises: acollector region having majority carriers of a first type; a base regionhaving majority carriers of a second type, the base region having adopant concentration greater than 1.0*10¹⁶ cm⁻³ at a depth of 0.2microns; an emitter region having majority carriers of the first type,the emitter region having a first area and a second area; silicide,wherein the silicide is formed in the second area of the emitter region;a gate oxide over the base region; and a gate material on the gateoxide, the gate material and the gate oxide having a first sidewall,wherein the silicide formed in the second area of the emitter region isseparated from the first sidewall by a distance of at least 0.1 microns.

In accordance with the third set of embodiments, in the transistor, thefirst area of the emitter region surrounds the second area of theemitter region, and the first sidewall surrounds the first area of theemitter region.

In accordance with a third set of embodiments, the transistor furthercomprises: a base contact region in the base region having majoritycarriers of the second type; wherein the base contact region has a firstarea and a second area; and wherein the silicide is formed in the secondarea of the base contact region.

In accordance with the third set of embodiments, in the transistor, thegate material and the gate oxide have a second sidewall, the second areaof the base contact region surrounds the first area of the base contactregion, and the first area of the base contact region surrounds thesecond sidewall.

In accordance with a third set of embodiments, the transistor furthercomprises a silicide block formed on the first sidewall and on the firstarea of the emitter region.

In accordance with the third set of embodiments, in the transistor, themajority carriers of the first type are holes and the majority carriersof the second type are electrons.

BRIEF DESCRIPTION OF THE DRAWINGS

For a detailed description of various examples, reference will now bemade to the accompanying drawings in which:

FIG. 1 shows an illustrative transistor in accordance with variousexamples;

FIG. 2 shows an illustrative transistor in accordance with variousexamples;

FIG. 3 shows an illustrative transistor in accordance with variousexamples;

FIG. 4 shows an illustrative transistor in accordance with variousexamples;

FIG. 5 shows an illustrative transistor in accordance with variousexamples; and

FIG. 6 shows an illustrative transistor fabrication process inaccordance with various examples.

DETAILED DESCRIPTION

In many high performance analog applications, a bipolar junctiontransistor (BJT) is often used to amplify, buffer, or condition ananalog signal. In such applications, it is desirable for a transistor tohave a relatively high beta Early voltage product, with relatively low1/f noise and popcorn noise. In current fabrication process technology,it can be difficult to achieve both of these design objectives. Asilicide block separating the emitter contact from the base contact in atransistor can be used to maintain a relatively high beta, but thisdesign does not mitigate the noise. To mitigate the noise, a gate oxide,protected by a polysilicon gate, can be disposed between the emittercontact and the base contact, but in current fabrication processtechnology the polysilicon gate defines the emitter contact area and thebase contact area, so that silicide formation on these areas may lead toa relatively low beta. It is desirable for a fabrication process andtransistor design to maintain a relatively high beta Early voltageproduct with relatively low 1/f noise and popcorn noise.

In accordance with the disclosed embodiments, a transistor is fabricatedin which a first implantation of dopants is performed to form a baseregion, followed by a second implantation of dopants into the baseregion. The first implantation of dopants provides a relatively deepimplantation to form the base region. The second implantation of dopantsprovides a relatively shallow implantation. The second implantation ofdopants increases the dopant concentration at a surface between anemitter region and a base contact region.

In accordance with the disclosed embodiments, in addition to fabricatingthe transistor with the first and second implantation of donors, thetransistor comprises a gate material on a gate oxide, disposed betweenan emitter region and a base contact region, where a dielectric isformed on a first sidewall of the gate material and the gate oxidebefore forming silicide. The dielectric formed on the first sidewallserves as a silicide block, preventing silicide from forming on thefirst sidewall, so that the silicide does not form on the entire area ofthe emitter region and is at a distance from the first sidewall. In someembodiments, the silicide formed on the emitter region is separated fromthe first sidewall by a distance of 0.1 microns to 1 microns. Asdiscussed below, the silicide block on the first sidewall helps reduce1/f noise and popcorn noise while maintaining a relatively hightransistor beta. In accordance with the disclosed embodiments, thedielectric is formed on a second sidewall of the gate material and thegate oxide before forming silicide.

FIG. 1 shows an illustrative transistor 100 according to an embodiment.The illustrative transistor 100 is a bipolar junction transistor (BJT),and includes a collector region 102, a base region 104, and an emitterregion 106. In some embodiments, the illustrative transistor 100 is aPNP transistor, where the collector region 102 and the emitter region106 are P-type semiconductors, and the base region 104 is an N-typesemiconductor. For a PNP transistor, the collector region 102 and theemitter region 106 can be fabricated by implanting acceptor dopants intoa silicon semiconductor, and the base region 104 can be fabricated byimplanting donor dopants into a silicon semiconductor.

In some embodiments, the illustrative transistor 100 is an NPNtransistor, where the collector region 102 and the emitter region 106are N-type semiconductors, and the base region 104 is a P-typesemiconductor. For an NPN transistor, the collector region 102 and theemitter region 106 can be fabricated by implanting donor dopants into asilicon semiconductor, and the base region 104 can be fabricated byimplanting acceptor dopants into a silicon semiconductor.

More generally, the collector region 102 and the emitter region 106 maybe described as having majority carriers of a first type, and the baseregion 104 may be described as having majority carriers of a secondtype. For some embodiments, holes can be the first type of majoritycarriers and electrons can be the second type of majority carriers.

FIG. 1 presents a cross-sectional slice of the illustrative transistor100, and is not drawn to scale. FIG. 1 does not show various vias andmetal layers that are fabricated in a back-end-of-line (BEOL) process toconnect the illustrative transistor 100 to other devices (not shown) toform a circuit.

FIG. 1 does not show the semiconductor substrate that the collectorregion 102 is formed in, nor does FIG. 1 show the semiconductorsubstrate as part of a wafer in which other devices may be integratedwith the illustrative transistor 100. The collector region 102 may befabricated within a well formed in the semiconductor substrate, andthere may be shallow trench isolation (STI) regions to isolate theillustrative transistor 100 from other devices (not shown). Thesemiconductor material upon which the illustrative transistor 100 isfabricated may be obtained from crystalline silicon grown from a seed,or the semiconductor material may also include epitaxial layers grownupon a semiconductor substrate.

FIG. 2 shows the illustrative transistor 100 according to an embodiment,where several of the components in FIG. 1 are shown in more detail. FIG.3 shows the illustrative transistor 100 according to an embodiment,where several of the components in FIG. 1 are shown in more detail, butwith respect to a different orientation of view.

A coordinate system 101 shown in FIG. 1, FIG. 2, and FIG. 3, shows therelationship among the orientations of views depicted in these figures.In FIG. 1, the coordinate system 101 has its x-axis and z-axis in thepage of the drawing, where the y-axis (not shown) points into the pageof the drawing. In this orientation, the x-y plane of the coordinatesystem 101 is parallel to the surface of the semiconductor upon whichthe illustrative transistor 100 is fabricated.

In FIG. 2, the coordinate system 101 has the same orientation as shownin FIG. 1. In FIG. 3, the coordinate system 101 has its x-axis andy-axis lying in the page of the drawing, where the z-axis (not shown)points out of the page of the drawing. The x-y plane of the coordinatesystem 101 as shown in FIG. 3 is still parallel to the surface of thesemiconductor upon which the illustrative transistor 100 is fabricated,but the orientation of view depicted in FIG. 3 may be described aslooking down upon the illustrative transistor 100. As for FIG. 1, FIG. 2and FIG. 3 do not show all components of the illustrative transistor 100that are fabricated for an operational circuit, and these figures arenot drawn to scale.

Referring to FIG. 2, the emitter region 106 has a first area 108 and asecond area 110. FIG. 2 does not depict the diffusion of the emitterregion 106 that would occur during fabrication. For example, in practicesome of the emitter region 106 would diffuse underneath the gate oxide114. In practice, the union of the first area 108 and the second area110 is somewhat less than the entire area of the emitter region 106,although FIG. 2 does not show this.

In some embodiments, the first area 108 surrounds the second area 110.FIG. 3 shows the first area 108 surrounding the second area 110. FIG. 3shows the first area 108 and the second area 110 of the emitter region106 as having rectangular boundaries, but this depiction is asimplification.

Referring to FIG. 2, silicide 202 is formed in the second area 110 ofthe emitter region 106. Depositing a metal onto the second area 110 ofthe emitter region 106, and then annealing forms the silicide 202. Themetal may comprise tungsten. The silicide 202 provides electricalconnection of the emitter region 106 to other circuit components (notshown).

Referring to FIG. 1, a base contact region 112 is formed in the baseregion 104 to provide ohmic contact to the base region 104. The basecontact region 112 may be formed by source-drain implantation. For theexample in which the illustrative transistor 100 is a PNP transistor,the base contact region 112 is an N-type semiconductor. For example,donor dopants may be implanted into the base region 104 to form the basecontact region 112. For the example in which the illustrative transistor100 is an NPN transistor, the base contact region 112 is a P-typesemiconductor. For example, acceptor dopants may be implanted into thebase region 104 to form the base contact region 112.

A gate oxide 114 is formed over the base region 104, illustrated in FIG.1 and FIG. 2. For some embodiments, the gate oxide 114 may comprisesilicon dioxide (SiO₂), and is a high-quality oxide thermally grown onthe semiconductor upon which the illustrative transistor 100 isfabricated.

Formed over the gate oxide 114 is a gate material 116, illustrated inFIG. 1 and FIG. 2. The gate material 116 may comprise polysilicon, andprotects the gate oxide 114 during subsequent processing steps thatcould damage the gate oxide 114 if the gate material 116 were notpresent. For some embodiments, the combination of the gate oxide 114 andthe gate material 116 surround the emitter region 106. This isillustrated in FIG. 3, showing the gate material 116 surrounding thefirst area 108 of the emitter region 106. (FIG. 3 does not show the gateoxide 114 because it lies underneath the gate material 116.)

The combination of the gate oxide 114 and the gate material 116 may bedescribed as being disposed between the emitter region 106 and the basecontact region 112. Isolating the emitter region 106 from the basecontact region 112 with a high quality oxide provided by the gate oxide114 helps with the 1/f noise and popcorn noise of the illustrativetransistor 100 during operation in a circuit, such as an analogamplifier.

FIG. 4 shows the illustrative transistor 100 according to an embodiment,where several of the components in FIG. 1 are shown in more detail. Thegate material 116 and the gate oxide 114 may be described as having afirst sidewall 402 and a second sidewall 404. The gate material 116 andthe gate oxide 114 can serve as a hard mask when implanting dopants toform the emitter region 106 and the base contact region 112, so that thefirst sidewall 402 may be viewed as defining a boundary of the emitterregion 106 and the second sidewall 404 may be viewed as defining aboundary of the base contact region 112.

Because of diffusion, the previous statement regarding the boundaries ofthe emitter region 106 and the base region 112 is only approximate, andthere is no precise definition to these boundaries, nor are theseboundaries exactly aligned with the sidewalls. Nevertheless, forpurposes of illustrating the embodiments, FIG. 3 shows a boundary 302 ofthe emitter region 106 that ideally is aligned with the first sidewall402, and a boundary 304 of the base contact region 112 that ideally isaligned with the second sidewall 404. The first sidewall 402 may bedescribed as surrounding the first area 108 of the emitter region 106.The boundary 304 of the base contact region 112 may be described assurrounding the second sidewall 404.

Referring to FIG. 1 (or FIG. 2), a silicide block 118 is formed on thefirst sidewall 402. Referring to FIG. 2, the silicide block 118 coversthe first area 108 of the emitter region 106. The silicide block 118leaves uncovered (or exposed) the second area 110 of the emitter region106. The silicide block 118 is deposited on the first sidewall 402before depositing metal to form the silicide 202. When annealing thesemiconductor with metal to form the silicide 202, the silicide block118 prevents silicide from forming on the first sidewall 402. Thesilicide block 118 is disposed between the silicide 202 and the firstsidewall 402.

The silicide block 118 comprises a dielectric, where metal deposited onthe silicide block 118 is prevented from forming a silicide with thesilicon directly beneath the silicide block 118. For some embodiments,the silicide block 118 comprises silicon dioxide, and is formed bydepositing silicon dioxide onto the surface of the semiconductor uponwhich the illustrative transistor 100 is fabricated. For someembodiments, the silicide block 118 is deposited over the entire surfaceof the semiconductor upon which the illustrative transistor 100 isfabricated, and is selectively etched away to cover the first sidewall402 (and other components if desired). For example, anisotropic etchingmay be performed so that some silicon dioxide remains on the firstsidewall 402.

The silicide block 118 restricts formation of the silicide 202 to thesecond area 110 rather than to the entire area of the emitter region106, if the silicide block 118 were not present. With the silicide block118 present before forming the silicide 202, formation of the silicide202 is kept at a distance from the first sidewall 402, where, for someembodiments, this distance may be from 0.1 microns to 1 microns.Referring to FIG. 1, it may be seen that for a PNP transistor, electronsinjected laterally from the base region 104 into the emitter region 106experience more acceptors on their way to the silicide 202 than if therewas silicide over the entire area of the emitter region 106. Acceptorspresent a potential barrier to electrons, and consequently the silicideblock 118 helps mitigate lateral injection of base current to theemitter region 106, thereby increasing transistor beta.

For a PNP transistor, some of the electrons injected vertically from thebase region 104 into the emitter region 106 may encounter the silicideblock 118, and are expected to be reflected back into the base region104, thereby further reducing base current and contributing to anincrease in transistor beta.

Referring to FIG. 1, dopants are implanted in the collector region 102to form a first well 120, and dopants are implanted in the first well120 to form a collector contact region 122. The collector contact region122 may be formed by source-drain implantation. An STI region 124 isformed between the collector contact region 122 and the base contactregion 112 to provide electrical isolation. For a PNP transistor, thefirst well 120 and the collector contact region 122 are P-typesemiconductors. For an NPN transistor, the first well 120 and thecollector contact region 122 are N-type semiconductors. A second well126 may be formed in the collector region 102 between the base contactregion 112 and the collector contact region 122 to provide electricalisolation.

FIG. 5 shows the illustrative transistor 100 according to an embodiment,where several of the components in FIG. 1 are shown in more detail. Asilicide block 502 is formed on the second sidewall 404. The silicideblock 502 may be formed when the silicide block 118 is formed. Forexample, silicon dioxide may be deposited over the semiconductor uponwhich the illustrative transistor 100 is fabricated, and the depositedsilicon dioxide is etched back anisotropically to leave silicon dioxideon the sidewalls.

The silicide block 502 covers a first area 504 of the base contactregion 112, leaving uncovered (exposed) a second area 506 of the basecontact region 112. Depositing metal followed by annealing forms asilicide 508 in the second area 506 of the base contact region 112.(FIG. 1 does not show the silicide 508.) Forming the silicide 508 may beconcurrent with forming the silicide 202. The silicide block 502 isdeposited on the second sidewall 404 before forming the silicide 508, sothat the silicide block 502 prevents silicide from forming in the firstarea 504 of the base contact region 112, and prevents silicide fromforming on the second sidewall 404. The silicide block 502 is disposedbetween the silicide 508 and the second sidewall 404.

Referring to FIG. 3, the second area 506 of the base contact region 112surrounds the first area 504 of the base contact region 112. Inpractice, due to diffusion the union of the first area 504 and thesecond area 506 is somewhat less than the entire area of the basecontact region 112, although for purposes of describing the embodiments,the union of the first area 504 and the second area 506 may be viewed asrepresenting the entire area of the base contact region 112.Accordingly, FIG. 3 shows that the base contact region 112 surrounds thegate material 116. Ideally, the boundary 304 of FIG. 3 is aligned withthe second sidewall 404, so that FIG. 3 shows that the first area 504 ofthe base contact region 112 surrounds the second sidewall 404.

To form the base region 104, dopants are implanted with a first dosageinto the collector region 102 to form a base region (not yet the finalbase region 104), followed by a second implantation of dopants with asecond dosage to form the base region 104. The first and second dosagesare such that the first implantation is deeper than the secondimplantation.

In some embodiments, the illustrative transistor 100 is an NPNtransistor, where implanting dopants with the first dosage (for a deepimplantation) comprises implanting boron at a dose of 5.0*10¹³ cm⁻² withan energy of 140 keV, and implanting dopants with the second dosage (fora shallow implantation) comprises implanting boron at a dose of 4.0*10¹³cm⁻² with an energy of 20 keV.

Adding the shallow implantation in addition to the deep implantationbrings about a higher concentration of dopants (e.g., acceptors for anNPN transistor and donors for a PNP transistor) at the surface betweenthe emitter region 106 and the base contact region 112. As an example ofthe higher concentration of boron due to the shallow implant, in someembodiments, the concentration of boron at a depth of 0.2 microns isgreater than 1.0*10¹⁶ cm⁻³.

The added concentration of dopants near the surface of the emitterregion 106 presents a potential barrier to minority carriers in the baseregion 104, thereby contributing to the beta. For example, for an NPNtransistor, electrons injected laterally from the emitter region 106experience a higher potential barrier because of the shallow implant.Consequently, there are less electrons being absorbed by the basecontact region 112, thereby reducing base current and resulting in ahigher beta.

FIG. 6 shows an illustrative process 600 for fabricating theillustrative transistor 100 according to an embodiment. In step 602,dopants are implanted in a semiconductor to form a collector regionhaving majority carriers of a first type. In step 603, dopants areimplanted with a first dosage in the collector region to form a baseregion. In step 604, dopants are implanted with a second dosage in thecollector region to form the base region having majority carriers of asecond type. In step 606, a gate oxide is grown on the semiconductor,and in step 608, a gate material is deposited on the gate oxide.

In step 610, the gate material and the gate oxide are etched to exposean emitter area of the base region, and to expose a base contact area inthe base region. The etching of the gate material and the gate oxideforms a first sidewall and a second sidewall. For some embodiments, thefirst sidewall of the gate material and the gate oxide surround theemitter area. (Other embodiments could be directed to fabricatinglateral transistors.) For some embodiments, the base contact areasurrounds the second sidewall of the gate material and the gate oxide.

The gate material and the gate oxide after etching can serve as a hardmask for defining the emitter area, as well as defining other areas forimplanting dopants. In step 612, dopants are implanted in the emitterarea to form an emitter region having majority carriers of the firsttype. In step 614, dopants are implanted in the base contact area toform a base contact region having majority carriers of the second type.

Steps 616 and 618 form the collector region of the transistor, where instep 616 dopants are implanted in the collector region to form a wellhaving majority carriers of the first type, and in step 618 dopants areimplanted in the well to form a collector contact drain region havingmajority carriers of the first type to make contact with the collectorregion.

Before implanting dopants, a photoresist film is deposited and exposedwith radiation by one or more lithography masks, followed by baking andetching of the photoresist film to define a pattern on the semiconductorfor the dopant implantation. However, such steps are not included inFIG. 6.

In step 620, a dielectric is deposited over the semiconductor. Forexample, silicon dioxide may be deposited by chemical vapor deposition(CVD). This dielectric serves as a silicide block. In step 622, thedielectric is etched to cover the first sidewall of the gate materialand the gate oxide. In step 624, metal is deposited over the surface ofthe semiconductor and the dielectric, followed by annealing in step 626so that the metal in contact with silicon forms silicide. In step 628,for some embodiments, the dielectric deposited in step 624 may be etchedso as to cover the second sidewall of the gate material and the gateoxide.

The listing of steps in FIG. 6 does not necessarily imply acorresponding ordering of the steps when fabricating a transistoraccording to an embodiment. However, the steps 620 and 622 to depositand etch dielectric to form a silicide block on the first sidewall isperformed before the steps of 624 and 626 to form silicide in theemitter region. Similarly, the steps 620 and 628 to deposit and etchdielectric to form a silicide block on the second sidewall is performedbefore the steps of 624 and 626 to form silicide in the base contactregion.

Furthermore, some embodiments may not include all steps listed in FIG.6. For example, some embodiments may not include a silicide block, sothat steps 620 and 622 need not be performed before step 624.

The above discussion is meant to be illustrative of the principles andvarious embodiments of the present invention. Numerous variations andmodifications will become apparent to those skilled in the art once theabove disclosure is fully appreciated. It is intended that the followingclaims be interpreted to embrace all such variations and modifications.

What is claimed is:
 1. A transistor comprising: a collector regionhaving majority carriers of a first type; a base region having majoritycarriers of a second type, the base region having a dopant concentrationgreater than 1.0*10¹⁶ cm⁻³ at a depth of 0.2 μm; an emitter regionhaving majority carriers of the first type, the emitter region having afirst area and a second area; silicide, wherein the silicide is formedin the second area of the emitter region; a gate oxide over the baseregion; and a gate material on the gate oxide, the gate material and thegate oxide having a first sidewall, wherein the silicide formed in thesecond area of the emitter region is separated from the first sidewallby a distance of at least 0.1 microns.
 2. The transistor of claim 1,wherein the first area of the emitter region surrounds the second areaof the emitter region, and the first sidewall surrounds the first areaof the emitter region.
 3. The transistor of claim 1, further comprising:a base contact region in the base region having majority carriers of thesecond type; wherein the base contact region has a first area and asecond area; and wherein the silicide is formed in the second area ofthe base contact region.
 4. The transistor of claim 3, wherein the gatematerial and the gate oxide have a second sidewall, the second area ofthe base contact region surrounds the first area of the base contactregion, and the first area of the base contact region surrounds thesecond sidewall.
 5. The transistor of claim 1, further comprising asilicide block formed on the first sidewall and on the first area of theemitter region.
 6. The transistor of claim 1, wherein the majoritycarriers of the first type are holes and the majority carriers of thesecond type are electrons.
 7. An integrated circuit, comprising: a firstdoped region having a first conductivity type formed in a semiconductorsubstrate; a second region having a second opposite conductivity typeformed within and laterally surrounded by the first doped region; athird doped region having the first conductivity type formed within andlaterally surrounded by the second doped region; a silicided portion ofthe third doped region spaced apart from the second doped region andlaterally surrounded at a surface of the substrate by an unsilicidedportion of the third doped region.
 8. The integrated circuit of claim 7,further comprising a spacer on the substrate surface between the thirddoped region and a doped contact to the second doped region, the dopedcontact having the second conductivity type.
 9. The integrated circuitof claim 8, wherein the spacer includes an oxide layer directly on thesecond doped region and a polysilicon spacer over the oxide layer. 10.The integrated circuit of claim 7, wherein the first and third dopedregions are N-type and the second doped region is P-type.
 11. Theintegrated circuit of claim 7, wherein the second doped region has adopant concentration greater than 1.0*10¹⁶ cm⁻³ at a depth of 0.2 μmbelow the substrate surface.
 12. The integrated circuit of claim 8,further comprising a sidewall dielectric on an interior sidewall of thespacer, the sidewall dielectric spacing apart the silicided portion fromthe second doped region.
 13. A bipolar junction transistor of anintegrated circuit, comprising: a collector region having majoritycarriers of first type formed in a semiconductor substrate; a baseregion having majority carriers of a second opposite type between thecollector region and a surface of the substrate, the base regionlaterally surrounded by the collector region; an emitter region havingmajority carriers of the first type between the base region and thesubstrate surface, the emitter region laterally surrounded by the baseregion; a silicide layer formed in the emitter region and spaced apartfrom the base region and laterally surrounded at the surface of thesubstrate by the emitter region.
 14. The bipolar junction transistor ofclaim 13, further comprising a gate dielectric ring on the substratesurface between the emitter region and a doped contact to the baseregion, the doped contact having majority carriers of the second type.15. The bipolar junction transistor of claim 14, further comprising apolysilicon spacer ring on the gate dielectric ring.
 16. The bipolarjunction transistor of claim 14, wherein the emitter and collectorregions are P-type and the base region is P-type.
 17. The bipolarjunction transistor of claim 14, wherein the base region has a dopantconcentration greater than 1.0*10¹⁶ cm⁻³ at a depth of 0.2 μm below thesubstrate surface.
 18. The bipolar junction transistor of claim 15,further comprising a sidewall spacer on an interior sidewall of thepolysilicon spacer ring, the sidewall spacer spacing apart the silicidelayer from the base region.
 19. The bipolar junction transistor of claim15, further comprising a sidewall spacer on an exterior sidewall of thepolysilicon spacer ring, the sidewall spacer spacing apart thepolysilicon spacer ring from a silicide contact to the base region.